The combustion process in an internal combustion engine produces NOx (principally NO and NO2), CO, CO2, HC (HydroCarbons), and PM (Particulate Matter).
The amount of CO2 depends on the amount of fuel injected into the cylinders and the amount of CO and HC depends on the combustion efficiency of the internal combustion engine. The amount of NOX depends on the combustion temperature and on the amount of oxygen introduced into the cylinders, while the amount of PM is strictly dependent on the air to fuel ratio (λ).
To optimize the amount of PM and NOX produced, combustion engines are provided with an EGR (Exhaust Gas Recirculation) circuit. The EGR system recirculates exhaust gas from the exhaust manifold to the intake manifold in order to dilute the fresh air introduced into the engine. This leads to emission optimization during the combustion process, because higher amount of H2O and CO2 are introduced, which have a high heat capacity that reduces the combustion temperature. Another effect of diluting the intake flow is that it is possible to control the amount of O2 in the intake flow. The counter effect of this system is that the more the fresh air is diluted, the more the air to fuel ratio (λ) is reduced. This leads to higher amount of PM emissions.
The quantity of fresh air and exhaust gas flowing into the cylinders is controlled by the throttle valve and the EGR valve, respectively.
In conventional engines there is an electronic control unit (ECU) arranged to control the position of the EGR valve in closed loop (if the EGR valve may be controlled with a position feedback loop) and to control the air or the oxygen quantity in the intake manifold, the EGR rate, the λ, the oxygen quantity in the exhaust manifold, by acting either on the throttle valve or on the EGR valve.
The ECU is also arranged to control the temperature of the gas flowing through the EGR circuit in open loop (in case of presence of a cooler bypass in the EGR system) and to control the flap position of the charging system (in case of presence of a charging system that may be controlled with a position feedback loop).
Moreover, the ECU is arranged to control the pressure downstream the charging system, if present, by means of a boost pressure controller, using signals coming from a pressure sensor.
Known air control systems are provided with an air controller and a boost pressure controller, contained in the ECU, that make the engine operate using predetermined fresh air set point and boost set point set by the ECU.
The inputs of the air controller are the fresh air set point and an actual value of fresh mass air flow entering the engine, measured by an air mass sensor placed at the inlet of a turbocharger. The inputs of the boost pressure controller are the boost set point and an actual value of boost pressure in the intake manifold, measured by a sensor placed in the intake manifold downstream the mixing point between the fresh air flow and the recirculated gas flow. The output of the air controller is an actuation request both for the EGR and the throttle valve, which is sent respectively to two different position controllers which control the opening of the two valves.
The drawbacks of such architecture is that the control of the throttle valve and the EGR valve are based on a single air set point, and this leads to difficulties in controlling the emissions and coordinating the two valves.
In view of the above, it is at least one object of the present invention to provide an improved method for controlling the EGR valve and the throttle valve, which allows for control of each valve independently so as to obtain a better emission control. In addition, other objects, desirable features, and characteristics will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background.